The foamy viruses (FV) subgroup of retroid viruses has attracted scientific interest because of their unique replication strategy and because of their potential use as gene transfer vectors (35). It has been proposed that FVs may be ideal tools for the development of a gene delivery system due to specific properties of this virus group, such as the absence of FV antibodies in the human population, the benign course of natural FV infections, their very broad host cell range, and an extended packaging limit due to the size of the FV genome (4, 30, 32). However, limited knowledge of the molecular biology of this virus group has so far not allowed the development of safe packaging cell lines and vectors, such as those that have been derived for murine retroviruses, among others (27). For instance, the FV genome is a double-stranded DNA with a complex organization. In addition to LTRs (Long Terminal Repeat), a packaging region and gag, pol, env genes, it also comprises several genes such as bel1, bel2, bel3, bet, beo and bes located between env and 3'LTR. The env gene encodes a 130 kDa glycosylated precursor that is cleaved giving rise to the surface (SU) and transmembrane (TM) proteins (see FIG. 1). Furthermore, FVs express their Pol protein from a spliced mRNA independently of the Gag protein, and the mechanism of FV genome packaging and particle assembly, as well as the significance of high amounts of reverse transcribed DNA in the extra-cellular particle are largely unknown (10, 18, 39). Other unique features include the nuclear localization of the Gag precursor protein (31, 40) and the predominant budding into intracytoplasmic vesicles which may be a consequence of the retention of the Env precursor protein in the ER (13).
Moloney retrovirus-based gene transfer vectors are currently the main vehicles for high efficiency stable gene transfer into a wide variety of cells types (20). Major limitations of this vector system are the restricted host-cell range and the inefficient infectivity for some human cells (reviewed in (1)). Recently, several methods using the pseudotyping with foreign envelope proteins, such as the vesicular stomatitis virus (VSV) G glycoprotein (6, 38) or the gibbon ape leukemia virus (GALV) envelope protein (2, 34) have been shown to overcome these disadvantages.
However, the expression of VSV-G for example is highly toxic for the producer cells and has prevented the generation of stable VSV-G packaging cells line (8, 22, 37).